A system for electrically heating a fluid being transported in a pipe

ABSTRACT

A system for heating a fluid being transported in a pipe includes a ferromagnetic heat-tube coextensive with a section of pipe to be heated. The heat-tube is secured in heat exchange relation to the pipe and has a substantial part of its wall in common with the wall of the pipe. An insulated conductor is disposed within the tube. One end of the conductor is electrically connected to one terminal of an AC source and the other end thereof is electrically connected to an end of the tube. The other end of the tube is electrically connected to the other terminal of the AC source. When alternating current flows through the circuit thus established, it passes only the inner surface of the tube due to the skin effect of the alternating current, thereby generating heat on the inner surface of the tube without current appearing on the outer surface thereof. The heattube can be formed by welding a concave strip to the outer cylinder surface of the pipe with the conductor disposed therebetween. Alternatively, the heat-tube can be integrally formed with the pipe as one channel of a two-channel pipe, the other channel being used to transport the fluid. The heat-tube may be helically wound around pipe. The heat-tube is either evacuated or pressurized and means to detect pressure changes indicative of heat-tube leakage is provided.

United States Patent 72] Inventor Donald F. Othmer 333 Jay St., Brooklyn, N.Y. 11201 [21] Appl. No. 805,718 [22] Filed Mar. 10, 1969 [.45] Patented Nov. 2, 1971 [54] A SYSTEM FOR ELECTRICALLY HEATING A FLUID BEING TRANSPORTED IN A PIPE 32 Claims, 8 Drawing Figs.

[52 US. Cl 219/300, 137/341, 219/535 [51] Int. Cl 1105b 3/00, F24j 3/04 [50] Field 01 Search 219/300,

FOREIGN PATENTS 1,546,486 10/1968 France 219/300 49,610 7/1921 Sweden 219/535 253,430 11/1948 Switzerland 219/301 Primary Exam iner-A. Bartis ABSTRACT: A system for heating a fluid being transported in a pipe includes a ferromagnetic heat-tube coextensive with a section of pipe to be heated. The heat-tube is secured in heat exchange relation to the pipe and has a substantial part of its wall in common with the wall of the pipe. An insulated conductor is disposed within the tube. One end of the conductor is electrically connected to one terminal of an AC source and the other end thereofis electrically connected to an end of the tube. The other end of the tube is electrically connected to the other terminal of the AC source. When alternating current flows through the circuit thus established, it passes only the inner surface ofthe tube due to the skin effect of the alternating current, thereby generating heat on the inner surface of the tube without current appearing on the outer surface thereof. The heat-tube can be formed by welding a concave strip to the outer cylinder surface of the pipe with the conductor disposed therebetween. Alternatively, the heat-tube can be integrally formed with the pipe as one channel of a two-channel pipe, the other channel being used to transport the fluid. The heat-tube may be helically wound around pipe. The heattube is either evacuated or pressurized and means to detect pressure changes indicative of heat-tube leakage is provided.

1 A SYSTEM FOR ELECTRICAIJLY HEATING A FLUID BEING TRANSPORTED IN A PIPE This invention relates to the use of the skin-effect of alternating current AC flowing with an adjacent or concentric steel conductor supplying the return or"back" leg of the circuit, thus causing induction and magnetic effects which greatly increase the effective resistance of the steel conductor.

Alternating current AC flows only along the skin of a steel conductor under these conditions. In a tube having a minimum wall thickness of about lone-sixteenth inch-less for many steels-and with AC carried out to the far end by an intemal, insulated wire, and back by the tube, due to what is called skin-effect," all AC flows back on the inside surface of skin of the tube and its outside is completely insulated electrically. This considerable reduction of what is normally re garded as the effective cross section of an electrical conductor greatly increases its effective resistance, so that steel tubes of substantial cross section of metal-compared to the usual copper wire conductor-offer greatly increased resistance and hence can be used for resistanceiheating with AC for which they would be quite unsuitable with direct currentDC It has been found that these tubes can be designed to give off considerable heat; and they may be used as heaters in many industrial and domestic operations. Since AC flows only on the inside surface of such a heat-tube, the outer wall of the steel pipe is perfectly insulated from the AC Itusually is grounded and may be touched without shock. The tube may be used directly as a pipeline, even of considerable size, for liquids to be kept heated in transit.

HEATING OF PIPELINES BY SIiIN-EFFECT AS IN PRIOR ART .even with the low performance of the prior art, has large ad.-

vantages over other systems which have been used, such as:

a. heating the pipe wall electrically and directly by normal impedance or resistance to current flow, usually with low voltage DC I b. heating of the pipe wall by a conventional electric resistor by attachment to its surface and sti'itablyinsulated therefrom;

c. heating of the pipe wall by a hot fluid, usually steam, in a small tube called a tracer tube" or trace" running along and in contact with an element of the pipeline.

For long pipelines, the considerable distances from a central source of thermal energy (such as a boiler) makes the connecting steam system for a tracer installation extensive and ex pensive. Instead, tracer tubes can be heated economically throughout their length by the skin-effect, usingAC, which has skin-effect phenomenon has allowed its economical use in a tracer tube so heated even though the BTUs of heat from electricity are usually more expensive than from steam or heated liquids circulated in a trace tube. Compared with steam: electricity may be transported long distances, controlled much more readily, and utilized much more efficiently. Thus, this method of heating of pipelines is superseding other methods, which are relatively expensive and clumsy in installation and maintenance.

The industrial practice of uslngthe skin-effect of 50 to 60 cycle AC for resistance heating has so far been limited to the heating of oil-pipes to reduce the viscosity of the oil being transported. The teachings of U.S. Pat. No. 3,293,407 have been utilized; and a relatively small steel heat-tube, %-inch standard-pipe size, has been specified.

This heat-tube has always been substantially axially parallel to the oil-pipe, with an internal insulated copper wire. This internal electric wire forms one leg out of the AC circuit; and its other terminal was at the far end of the heattube on the inside thereof. Electric current flows back on the return leg of the circuit on the inside wall of the heat-tube because of the skineffect, with no current flowing on the outside wall, if the steel tube is more than about 0.04-inches thick. The other junction to the source of AC is a point at the near end of the inner surface of the heat-tube adjacent the entrance of the insulated copper wire into theheattube to carry the AC to its far end.

Theheat-tubehas always been attached in substantially axially parallel relation with the oil-pipe and along one of its elements by welding to this much larger pipe which carries the fluid. Temperatures of theheat-tube more than a few degrees Fahrenheit higher than that of the oil-pipe could not be tolerated because expansion strains set up cracked off this type of attachment The standard thermal, insulation around the main pipeline, also covered the small heat-tube to minimize heat losses; but this insulation had to be applied in specially cut pieces, around the one to a halfdozen heabtubes built out as appendages on thesurfaces of the oil-pipe.

In the construction of the prior art, the. Outer walls of the oilpipe and of the heat-tube were two circles in contact and they were tangent, i.e., formed an angle of 0 at the point of contact. Welding might offer a small fillet. Thus, heat transfer has not been good from heattube to oil-pipe; heat fluxes have been low, particularly because of the low-temperature difference allowable between heat-tube and oil-pipe Also,thermal insulation hasbeen difficult and expensive to install.

In an alternate construction, less desirable because of inaccessibility, poorer heat distribution and greater pumping costs of the oil, the heat-tube, again in substantially axially parallel relation has been installed inside of the. oil-pipe supported along the axis by sets of three braces at suitable distances,

However, only a small heat flux has been generated by skineffect heating in either of these systems used to date-not more than 10 to 15 watts per lineal foot of the heat-tubes. This has required a large number of heat-tubes applied to theoutside surface ofevena moderate size pipeline, even in very moderate winter temperatures of 32 F 3 heat-tubcs for a 12- inchline, and ,6for a 30-inch line.

THE PRESENT INVENTION Methods have nowbeen found to increase the heat flux supplied to oil-pipes and other adjacent materials through the use of heat-tubes by an order of magnitude and to make possible one instead of six or more may be necessary; also it reduces pumping costs of the oil. Furthermore, considerable savings in thermal insulation costs are made possible. While the improved systems of this invention singly or-together do make possible these much greater heat fluxes, some of them also may be usedto great advantage in those installationswhere low heat fluxes will suffice.

Improved designs of the heating tube have been devised by making the, heat tube form an integral partof the oil-pipe or its wall. These new designs allow much greater heat fluxes than formerly thought possible, with important advantages and economies. These new types of heat-tubes also decrease greatly the costs of pipe insulation.

It has been found that, by correct design, the AC may be made to travel several times the length of the pipe through spiral-wound heat-tubes, rather than going through the straight line path of what would normally .be quite the least resistance. These spiral-wound heat-tubes thus made possible have now been found to be cheaper in both installation and operating costs, and to allow a very much greater heat flux per; unit length of-heat-tube than the longitudinal heat-tubes used,

to date, while substantially reducing pumping costs in the oilpipe line due to more uniform heating of the pipe and the oil; While they may be used at any low heat flux, they are particularly advantageous when using the high heat flux of this invention.

These new developments, and others, all a part of this invention, by creating greater heat fluxes, allow larger pipelines for transporting oil, particularly viscous oils such as crudes or residues, to operate in colder ambient conditions and to be heated more economically by the use of:

a. new designs of the heat-tube as an integral part of the wall of the pipe. The angle of the wall of the heat-tube section with that of the oil-pipe has been found best when it is at least 90,

- and preferably more. By entirely burying the heat-tube in the wall of the oil-pipe. and sometimes partly in the internal fluid, this angle reaches the optimum, i.e., 180;

b. higher temperatures of the heated oil-pipes c. larger heat-tubes than heretofore thought possible, i.e., as large as 3 /-inches standard-iron pipe instead of Winch standard pipe;

d. higher operating temperatures of the heat-tubes, and particularly much higher differences of temperature between the heat-tube and the oil-pipe;

e. spirally wound heat-tubes, even though the length of the AC path through the wall of the steel pipe is thus increased several times;

f. improved forms of heatproof insulation of the internal conductor.

By these and other means to be described, it is now possible to increase the total heat dissipation to as high as 100 to 500 and in some cases even 200 to 500 watts per lineal foot of heat-tube, as contrasted with the to watts of the prior art. This high heat flux makes possible the installation of much larger pipelines in very cold climates, at a much lower cost and without an excessive number of heat-tubes for each length.

Also, it makes possible the operation of such long pipeline systems with much longer heat-tubes; hence fewer repeater stations, each of which is costly to build and particularly costly to man and to operate. Other economies of costs of installation and of operation will be made evident by the figures and their description.

As another feature of this invention, it is possible to monitor for leaks which might be developed in the heat-tube during service life by evacuating or by applying oil under pressure to the interconnected heat-tube system and noting any change in pressure on a gauge connected thereto.

FUNDAMENTALS OF THE INVENTION Skin-Effect Caused by Induced magnetic Flux Skin-efiect is a phenomenon of AC, which restricts the flow of AC to the surfaces of some metal conductors exposed to electromagnetic (EM) fields. Iron and steel conductors-resistors are affected at commercial AC frequencies of 50 to 60 cycles per second, when another adjacent conductor carries AC so as to generate surface magnetic and induction effects with corresponding diffusional functions of the AC in these ferromagnetic materials. In some cases, all three phases of standard AC current generation may be used to advantage.

The electro magnetic flux surrounding a wire carrying an AC has been found to extend without practical diminution of its influence on the skin-effect for some distance, if not shielded by another metal. Thus, it has been found possible to use a larger individual heat-tube than before.

The use of the larger individual heat-tubes allows a much greater elTective conductor-resistor, i.e., the inner skin of the larger heat-tube. Particularly it allows a heavier internal insulated electric wire, which larger size wire is necessary for carrying the high voltage and intensity AC required for heating long distance pipelines. Also, the larger heat-tube allows the use of a poorer conductor for the AC, hence of larger diameter for the same intensity of AC than the usual copper wire.

Where the terms skin" and skin-effect" are used, these are not absolute terms. There is a great tendency for AC flow to be near the inner surface of the wall of a tube having an axial conductor of AC, herein called a heat-tube;" and the current density falls off in the ube wall according to an exponential function of the distance Irom the inner surface.

Under such conditions, and with a commercial AC with a frequency of 50 or 60 cycles per second, it has been found that the effective thickness or depth, a, of the skin through which AC is flowing, is directly proportional to the square root of the resistivity, p, of the metal under the particular conditions and inversely proportional to the square root of the magnetic permeability, p, or

oconstant XNp'fI'p. From this, a useful relation is developed. The efl'ective resistance in any conductor, R, is directly proportional to the specific resistivity, or

R =constant x p The effective resistance of a conductor having skin-effect is inversely proportional to the effective cross sectional area of the skin conductor. This cross sectional area for a heat-tube is directly proportional to the effective skin depth; thus R is proportional to or, in heat-tubes:

- R constant XV Thus, while the electrical energy iii put and the heat output is, as always, directly proportional to the resistance of the conductor, which usually depends directly on the resistivity of the material of construction, the resistance and hence the heating effect of a heat-tube is proportional only to the square root of the resistivity of its material of construction.

Variations in this skin-effect are influenced by changes in the resistivity and the magnetic permeability which are caused by the temperature of the conductor. The gradation of current density against wall thickness from the inner surface of a small heat-tube is so large that, with the voltage drop experienced with steel tubes in heat-tube service (i.e., usually between 0.05 and 1.0 volts per lineal foot) and at temperatures up to about 400 to 500 F., the effective zero of current flow or availability is reached within a depth somewhat less than approximately onesixteenth inch from the inside tube surface. For most mild steels worked with, this depth has been found to be between 0.025 inch and 0.075 inch. For any particular steel, the effective conductor area is thus the inner perimeter of the heattube times a value of the depth, 5, between 0.025 inch and 0.075 inch.

in the outer part of the wall beyond this skin" on the inner surface of the tube, there is practically no current flow; and that part of the tube near the outer wall may be regarded as practically insulated from this AC flow a small fraction of an inch away. To be sure that there is no current leakage or danger from the high voltage AC flowing on the inside skin, the thickness of the tube from the innner to the outer wall should be about twice the skin thickness, or one-eighth inch under the usual intensities of AC flow used. There will then be no measurable voltage or power loss, even when the outside of the heat-tube is grounded or submerged in salt water. Unburied pipelines are grounded at reasonable distances, and pipelines in corrosive conditions may have the conventional sacrificial cathodic protection system with no interference with the skin-effect heating.

For heat-tubes of the same metal and same outside diameter, the one with the greatest wall thickness, and hence least internal diameter or internal perimeter, has been found to have the greatest skin-effect and the greatest resistance to flow of AC.

Materials for Heat-Tubes Other magnetic metals than carbon steel, usually other alloys of iron, also exhibit this skin-effect and may be used in the systems of this invention. Usual conductors such as copper, brass, and aluminum also exhibit a slight skin-effect but require frequencies of many more cycles per second to reduce the effective permeability or skin depth. Such high frequency AC is considerably more expensive to generate than the standard 50 to 60 cycle.

A metal which has properties which give effectively only a thin skin for AC conductance gives the least effective conducting cross-sectional area, and hence the greatest resistance. Metals of desired characteristics may be selected from the above-indicated relation of skin thickness as dependo have a relatively pronounced skin-effect, i.e., a thin skin ofconductance for AC under conditions of the present invention are: very pure iron; iron-nickel alloys, as ,l-lipernikythose with a small amount of manganese, called Permalloys; and

those with molybdenum, such as Superalloy. These have from four to six times the resistivity, and a thinner skinfor conductance ofAC in comparison with ordinary-mildsteel. Compared with ordinary steel, all of these are more expensive some are many times as costly. Thus, they are not v to be considered except for veryspecial applications.

While the term steel" is used herein to described ,the material .of construction of thewheat-tube, it may. be -..understood that this term formildor-ordinary carbon steel is used only as an example; and other metals, bothferrousand nonferrous, may also be used. Usually they have a less-desira ble skin-effect, or are more expensive. In some particular instances other physical or chemical properties ofother metallic conductors makethemworthy of consideration; but carbon steel is preferred for its cheapne ss, workability, and availability in many forms. Thus, the word steeli isused as, anexample without being a limitation of the material-of construction-of the heat-tubes of this invention.

Most pipelines for commercial liquidsare of steel or other metals, and the principal use offbeat-tubes with the high,heat

flux and as described by thisinvention is in heatingsuch pipes,

Many fluids have low viscosities at ambient temperatures.

Besides petroleum oils (crudes, idistillates, or residues), com- .mercial fluids transported by pipe, include molasses, other food syrups or melts-such as butter, other oils and fats, chocolate, etc-strongsulfuric.acid, tar, bitumen and many others.

Some materialsare frozen solid at some, ambient temperatures; in particular, there may-be noted water and aqueous solutions, sulfur, also benzene, acetic acid, etc. These-mustbe kept heated to prevent congealing or freezing if the ambient temperature is below the respective solidification temperature and sufficient heat above the freezing point cannot beadded before the fluid entersthe cold. length of pipe.. With acetic acid, the pipe may be of aluminum, ;stainless, steel, or copper-because of corrosion of carbon steel. The heat-tube would be of steel and particularcare would be taken that'its thickness be at least twice that of the skinor penetration depth of the AC. Most desirable would be a stainless steel; in other cases, it may be that ice and snow, with or without liquid water being present, fall within the classification of fluids to .be heated or melted, because of their being adjacent to the heattube.

Also, in handling some vapors or gases, it is desired to keep them in the vapor state by heating to a temperature above the ambient and above their dew.point, sothat their condensation is prevented. For example, it has been found that hydrochloric acid in gaseous form with water vapor and air, if present, may be piped in a steel pipe if the pipe is always at a temperature above the condensing pint of water, i.e., 2 l 2' F. at atmospheric pressure. Gaseous HCl does not attack steel, only is it corrosive if it dissolved in the waterwhich condenses on the inside of the pipeat temperatures below 212 F. In handling other vapors, it .may be desired also to superheat them above their boiling point at the particular pressure of the pipeline.

All of these, and many other fluids, may be heated to a temperature above the ambient, by the methods of this invention,

. particularly through-the high heat flux made possible. Tem- ,peratures as high or-higher than400" to 500 F. may be reached in an "oil-pipe.

' Elements of the Heat-Tube The heat-tube may be of a shape. other than cylindrical and have varied configurations in respect to the oil-pipe or other structuretobe heated; or it may be an integral part thereof. However, it is referred to here simply as the heat-tube" regardlessof its cross-sectional shape-.or convolutions, whether it ism'ade of several sections formed together longitudinally,

or even laterallyyor is a unitary tube.

Also, the electricconductor' for the one side of the AC circuit, ifcarried inside of'the heat-tube in whatever configuration-that may take, may be a copperwire in most cases-orjof other commercial metals or alloys. it may be of single or multi- -ple strands-of any desired arrangement-or cross'section. It is referred to usually simply as the electric-wire."

,Theelectric wire may bemade of other than usual metals.

.Metallicsodium may be-used as the conductor. A lining tube and hence is utilized in heating the adjacent materials, ,e.g.,' if attached to anoil-pipeto the oil therein.

Thus,"if.the electric wire is of copper, steel, or other metal of. greater resistivity, the additional heat which it givesup due to thelarger-line loss willall be utilized in the fundamental heatingpperation. Thus, a relatively inexpensive steel conductor or wire may be used in place of the more expensive but standard copperrllowever, theheatso generated within the wire willhave to pass through the electrical insulation,thence materials.

throughany. air space between the insulation and the heattube before it: adds to the heat from theheat-tube to be passed .to the surrounding materials. THe electric wire will usually have, a somewhat higher temperaturev than that of'theheat tube;. and thisnmust be considered in specifying insulation .lnsulation for the electric wire-may. be of anyv suitable material whichwill maintain its physical, electrical, and chemicalgcharacteristics as the temperature of the heat-tube.

' It is specified depending on its operating temperature: Polyvinyl chlorideis satisfactoryup to about 180 F., polyethylene up to about. 215 F., and specially cross-linked polyethylene I up,to about 260203". Silicon resin materials are available to be used from 350-400 F. Higher temperatures up to 500 F. or

above mayuse the insulation made .ofchloro-fluoro hydrocar- -bon resins commercially available at much higher prices.

The electric wire was not more than afraction of a' degree warmer than the heat-tube in the prior art. Thus, thecheapest resins-fort-insulation, PVC and Polyethylene, give adequate heat transfer withoutincreasing the cost'by expensive insula- .tion materials."Particularlyis this true in most cases of the .presentinvention because of the much better heat conduction .was limited by :the former art to a relatively few degrees-above that of the oil-pipe and not much more than that above the ambient,'it hasbeen found that the new designs of the-lteattube allowit to be used up toat least 400,F. or 500 F. with special materials even highenThus very large heat flux and heat input tothe adjacent materials being heated arepossible. By using different of the novel systems of this invention for the heat-tube, any desired difference up to I00 F. or evenmore,

between the heat-tube and the oil-pipe, may be used satisfactorily. Higher heat-tube temperatures are not usually necessary to secure the large advantages of the high heat fluxes of this invention-but an upper limit may be about 500 F. where the magnetic properties of the steel may change.

However, with the higher heat fluxes made possible by the present invention, the electric wire may operate at higher temperatures; and ceramic and other special inorganic insulations may be used in powder, cement, or bead form for these higher temperatures.

Special inorganic powders have been found to be useful for this purpose, particularly the oxides of the alkaline earth metals such as Beryllium, Magnesium, and Calcium. These oxides, such as magnesia, may be incorporated as an insulator inside the heat-tube and around the electric wire if of copper or particularly if of iron or other material of greater resistivity than copper. This powder must be firmly packed with the wire correctly aligned in the center of the tube in a factory operation. To compact adequately the insulation powder so that its heat conductivity will be improved, the assembled heat-tube, insulation, and wire may be passed hot or cold through rolls to reduce the size of the heat-tube slightly. A notable one of these oxides in powder form is beryllium oxide, which has excellent electrical insulation properties, while being a good heat conductor. it, like magnesium oxide, or calcium oxide, may be used in the upper limit of effectiveness of heat-tubes of the high heat fluxes of the present invention, but is too expensive for most uses. The heat tube preformed with the electric wire and its heatproof insulation, may then be brazed or welded into the oil-pipe in the shop or in the field by one of the several designs of this invention.

FIGURES in the equipment of this invention and in the figures as drawn, only AC is used; the conventional signs plus and minus indicate simply the two terminals for connection to the supply of AC by alternator or transformer. These symbols represent the direction of flow at only one instant of one cycle of the AC, which is reversed many times per second.

Thermal losses are minimized by application of conventional insulation materials in the usual manner to pipelines heated by this or other methods. Such insulation is not shown in the Figures as it is not a part of this invention, by itself. However, one of the major objects of this invention as applied to the heating of pipelines is the improvement of the ease and economy of application of insulation, because the rough contours of a small heat-tube (or several or more such) on the periphery of the large oil-pipe have been largely eliminated. insulation is a very substantial part of the cost of a large pipeline. By making the periphery'of the heat-tube when combined with the oil-pipe deviate as little as possible from the general circular cross-section, as is done by the new systems of heat-tubes now possible, the costs of labor and material for application of insulation are reduced very considerably. Besides this reduction of costs, the firmness, strength and life of the insulation covering is greatly increased; and maintenance or replacement costs are reduced or eliminated.

All figures are to be regarded as diagrammatic-no scale is followed. In particular, for ease of representation, the ratio of the size of a heat-tube to its oil-pipe is usually somewhat larger as drawn than it would be in commercial pipelines, particularly if the oil-pipes are large. Also, the electrical wiring connections are merely basic circuit indications.

Especially it should be noted that those representations of sections of long heat-tubes might continue for tens of thousands of times the diameter, although conventional breaks are not indicated.

HEAT-TUBES AND OIL-PIPES Heretotore, as in [1.8. Pat. No. 3,293,407, the practice has been to carry the electric wire through the heat-tubes made of standard iron pipe which is small in diameter in relation to that of the oil-pipe. This heat-tube has been welded to the surface of the oil-pipe. This may be a single appendage or protuberance on a small oil-pipe 4 to 10 inches in diameter, or as many as 6 or more such heat-tubes and protuberances on a pipeline of 30 inches or more in diameter. Such heat-tubes interfere greatly with the application of thermal insulation of the oil-pipe, and add greatly to such costs; also they give relatively poor contact for thermal conduction from the heat-tube to the oil-pipe. The angle of contact of the oil-pipe with the heattube is 0, i.e., they are tangent to each other. Because of the poor contact, only low-heat fluxes per unit length or per foot of internal perimeter of the oil may be used without excessive temperatures of the heat-tube. These high temperatures mean higher thermal losses, particularly since effective insulation is difficult.

The oil-pipe to be heated may be of any desired size; and those to be considered in practice range from 1 to 48 inches in diameter. The size of the heat-tube made of the steel used for conventional pipelines, depends on several conditions, principally: heat input required to attain or maintain the desired temperature of the contents of the oil-pipe of given size and flow, the AC voltage available, and the length. The wall should be at least %-inch thick for electrical, mechanical, and corrosion considerations.

The size of the heat-tube to be used, in all practical cases where the diameters are greater than about 4 times A, the depth of the effective conducting skin has been found thus to depend on its internal perimeter, not on its internal cross section, as it would be if carrying a fluid-and not on its wall cross section as it would be if carrying a DC current as a conventional electric conductor. However, the essential criterion of the inside perimeter or inside surface per unit length is not always attained most advantageously by the circular tube which is usually most readily available and at the lowest cost.

The surface of the oil-pipe may, in some cases, be used as part of the inner surface of the heat-tube, and this saves total weight of metal used. Furthermore, if the heat-tube is thus made an integral part of the wall of the oil-pipe, a part of the effective wall of the heat-tube is in contact with the oil being transported. Such considerations as good thermal contact and ease of insulation have been found to be important factors of optimum design, to allow good heat transfer.

The ratio of the inside perimeter of the heat-tube (or sums of perimeters if more than one heat-tube) to the outside perimeter of the oil-pipe, indicates roughly the ratio of the surface of electrical resistance heat input to the surface of heat output to the surroundings; i.e., heat losses of the system under conditions of constant temperature of the oil. The ratios as indicated by FIGS. 1 to 4 are not the optimum for any particular design conditions, since the figures are drawn without scale. in the past art, this ratio of the inside perimeter of the heat-tube (or the sum of the inside perimeters of all of the heat-tubes, if more than one is used,) to the outside perimeter of the oil-pipe has been in the range of about one-half to onefifth for moderate heat duties.

Because of the much greater rate of heat transfer or heat flux from the heat-tubes of the present invention, this ratio may be reduced to 1/7 to 1/10 or even 1/20 under comparable moderate heating conditions. This is of considerable importance under such circumstances as require to 200 watts or more of heat per foot of oil-pipe length, e.g., a 48- inch line under -50 F. ambient temperature.

For usual operations of pipelines carrying crude or other viscous petroleum oils, the electrical heat input supplied from the AC ofthe prior art was about 10 to 15 watts per lineal foot of the heat-tube. Higher heat inputs to the tube now increase greatly the temperature difference between the electric wire and the wall of the oil-pipe above the 3 to 4 F. regarded as a desirable maximum-usual operation desired was in the 2 to 3 F. range of temperature difference between the two walls. Higher temperatures of the heat-tubes now possible because of improved designs, including their integration into the oilpipe and the higher heat flux allow temperature differences between electric wire and oil-pipe wall of over 100 F. and the operation of large pipelines under severe winter conditions. The heat passed from the heat-tube around the periphery of the oil-pipe in the heat-tubes as presently designed, gives improved heat distribution without local overheating and underheating, consequent heat waste, and additional pumping costs.

The preferred designs of heat-tubes to be described have been found to improve greatly the heat flow from tube to pipe, and to minimize the temperature difference between them. They are useful under low heat flux conditions for their many advantages, but particularly, they make possible much higher heat fluxes. Thus, in oil-pipes up to 30 inches in diameter, only one such heat-tube parallel to the axis is necessary usually instead of thetwo to six of the prior art, not more than two are needed in sizes up to 48 inches, usually not more than three in sizes over 48 inches.

Besides allowing heat-tubes of relatively greater internal perimeters (from two to five times), these designs also allow greater input of electrical heat from an adequate source of AC-frorn I to 200 or even up to 500 watts per lineal foot of the heat-tube, while still not exceeding a safe difference of temperatures of the heattube and the adjacent oil-pipe in the preferred designs of the present invention. By the use of the preferred systems of this invention, the overall heat transfer per unit length may be to 30 times as much as by the prior art heat-tubes.

When using heat-tubes with oil-pipes, it is, as always, desirable to make at the far end of the heat-tube the connection of the electric wire with the inside of the tube. At the near end of the tube, it is desirable to make the other connection-going to the source of AC--to the inside of the tube. In both cases, it has been found that the connections may be made through the tube wall or to its outer surface, with an accompanying small local area of high voltage on the tribe's outer surface. Usually, this will be insulated; and it will not interfere with nearby grounding of that section of the oil-pipe. Such grounding is always accomplished periodically along the length of the oilpipe.

USE OF EXTERNAL SURFACE OFOIL-PIPE AS PART OF INTERNAL SURFACE OF HEAT-TUBE Mild carbon steel oil-pipes are most often used; and in the usual cases where the material of the oil pipe exerts the desired skin-effect, due to its properties of desired permeability and resistivity, some part of the surface of either a standard or a specially formed oil-pipe may be used as a part of the internal perimeter of the heat-tube.

FIG. 1 shows a made-in-place heat-tube 2, having the cross section of a lune. It is constructed by welding, 5, along its two edges a strip of one-eighth inch or thicker steel against two elements of the cylindrical oilpile, l. The steel strip is preformed as a trough with a radius of curvature substantially less than that of the outer surface of the heat-tube. The outer surface of the oil-pipe itself, between the two elements, becomes an effective part of the heat-tube and carries part of the AC by the skin-effect conduction on this part of its outer surface. Its skin resistance to the AC gives heat directly to the oil-pipe. The weight added to the pipeline by the heat-tube is less for the same internal perimeter. As much as 40 to 45 percent of the heat-tube is thus made up of the oil-pipe surface, which surface performs a dual service. Here, the built-up heattube section again approaches an angle of 180, with the surface of the oil-pipe.

The electric wire, 3, is indicated inside the heat-tube, 2, of FIG. I. as of special flattened cross section. Alternately, one, two, or more conventional insulated wires may be used to conduct the necessary AC. 1

In FIG. 2(a) is shown a modification of the system of FIG. 1, which has a better shape for accommodating a large circular electric wire, 3, with normal insulation, 4. The closer conformance of the shaped strip, 2, to the periphery of the oilpipe, 1, allows better heat transfer and some advantage in welding.

Another design of heat-tube is shown in FIG. 2(b), as if applied to the same oil-pipe, l. A special groove, 31, is formed by rolling or otherwise in the wall of l, and this is covered by a strip 2, of steel at least one-eighth inch thick, welded in place along both edges. This provides a heat-tube having an even larger part of its inner periphery made by the outer surface of the oil-pipe and with excellent heat transfer relation to the fluid inside, and an outer surface of the combination hardly disturbed from the circular. Thus, it is readily insulated. While the groove, 31, is shown deep enough to include entirely the electric wire, 3, a shallower groove and a concave formed cover strip may also be used.

Still another variation of the oil-pipe, somewhat easier to roll, is shown in FIG. 2(c) with a narrow flattened longitudinal section, 21, of the oil-pipe wall specially rolled during the production of the tube, and a formed strip 2 welded over it throughout the length, with the welded metal forming beams 5 on either side of 2. The usual conductor 3 has suitable insulation 4.

In each of the three heat-tubes of FIG. 2, the applied section approaches an angle of 180, with the outer surface of the oilpipe.

HEAT-TUBE AS ONE CHANNEL OF TWO CHANNEL PIPE FIG. 3 shows the cross section of an oilpipe, l, with a heattube, 2, built into it in the seamless drawing and forming of the pipe. Such dual duct pipes are available in small pipe sizes, usually in aluminum, which may not be used for this purpose. The common wall, 41, of the heat-tube and the oil-pipe allows excellent heat transfer and the unbroken cylindrical outer surface simplifies insulation. The critical angle is 180", since the common wall intersects the surface element to which may be drawn a tangent.

Another variation of the manufacture of the type of dualchannel tube of FIG. 3 is made with rolled pipe, wherein a larger pipe, at or above the welding temperature, and a small tube, at a somewhat lower temperature, are run through forming rolls which ultimately discharges a section not unlike that of FIG. 3, except that the common wall 41 is somewhat thicker than the outer wall of the oil-pipe. Usually a much smaller tube, 2, in relation to the oil-pipe, 1, might be used than that shown in FIG. 3.

This two-channel pipe is probably the optimum design for those smaller sizes of oil-pipe which may be so made, but suffcient lengths of pipe of a fixed size would have to be ordered to justify a pipe mill to set up for the special rolling or other forming operations.

As in other cases, mild steel will be the desirable material of construction of this type of heat tube, as one channel with the oil-pipe as the other.

HEAT-TUBE INTEGRALLY WELDED INTO PIPE WALL A preferred design to give the advantages of the system of FIG. 3 for larger pipe sizes, 12 inches and above, and particularly larger than 24 inches, is shown in FIG. 4(a), wherein the heat-tube, 2, is integrally welded into the wall of the oil-pipe,

l. The oil-pipe, 1, is formed of skelp which is pressed and/or rolled into a pipe which is not closed and has a slit left as an opening between the adjacent edges of the skelp. The heattube is inserted in this opening between the edges of the skelp. Standard pipemaking equipment welds, 5, along these common elements of the oil-pipe and the heat-tube. Selection of the size of the heat-tube is possible to vary design factors.

The heat-tube, 2 of FIG. 4(b), has been welded in the same way as was heat-tube, 2, of FIG. 4(a) except that it is supported during welding so that its outer element is flush with the surface of the oil-pipe, with the weld-head, 5, filling the space to give a surface which may be ground as a true cylinder if desired. Much of the heat-tube is thus inside the oil-pipe;

iii

and its surface gives direct and excellent heat transfer to the fluid. Another variation which may be used is to weld the heattube in as in 2 of FIG. 4(a); and after the welding, 2, is flattened by rolling or otherwise to conform almost exactly with the outside of the oil-pipe. However, the part inside the pipe wall disturbs, to some extent, the flow of the oil, and to minimize this disturbance, the heat-tube may be welded into the oil-pipe with one element flush with the inner surface of the oil-pipe. Neither of these slight variations are shown in the figures.

Since the outer surface of the heat-tube so constructed has an angle above 90, usually approaching or equaling [80 with the outer wall of the oil-pipe, it has excellent thermal relationship with the wall of the pipe; and it, in itself, is also in immediate contact with the fluid being transported. Such heattubes pass high heat fluxes, and offer little nuisance in application of insulation.

Large pipes, over 30 inches in diameter, frequently are made by pressing strips of skelp 40 feet to 60 feet long in dies to form semicircular troughs. These are made into pipes by welding two seams which become the opposite elements of the finished cylinder. For such oil-pipes of either larger or smaller diameter than 30 inches, two heat-tubes may be installed in the finished welding operations when making these semicircles into a full circle. Three or four heat-tubes may similarly be applied in seams of very large pipes, if pieces of skelp are cut, pressed, and welded with heat-tubes in each of the seams between pieces of skelp.

in large pipelines for transport of oil of the prior art, as many as six heat-tubes have been used under moderate conditions and even more would be necessary to give a distribution of the heat to the wall of an oil-pipe as large as 32 inches, and hence to its contents under severe conditions of heat losses due to low ambient temperatures. With the improved heat transfer of the heat-tube arrangements of this invention, such a large number is unnecessary for any oil-pipe of any size used in practice. However, any such number may be applied in the manner shown in FIGS. ll to 4, where particular design conditions indicate such to be desirable. For example, three such tubes may be used to carry respectively one each of the three phases from a standard alternator FIG. 4(a) or (b). If the design is used, each piece of skelp would then make up 120 of the finished pipe wall; and, as always, there would be the same number of seams and of heat-tubes as of pieces of skelp. The integration of the heat-tube into the wall of the oil-pipe makes possible the use of very high thermal fluxes; but other advantages of this system warrant its use even with lower thermal duties.

HEAT-TUBES NOT PARALLEL TO AXlS OF OlL-PlPE SPIRAL OR HELIX It has been found in oil-pipes for heavy grades of oil, and also in some other services, that the limit of the rate of heat transfer from one heat-tube, applied on an element of the pipe, is not reached before there may be an overheating of the oil in contact with that part of the oil-g ve carrying the heattube and with imperfect distribution of the heat to all parts of the fluid throughout the cross section of the pipe. The higher viscosity of that oil which is not so well heated is thus not overcome adequately, because of its lower temperature.

As a bad example might be the case of the heat-tube running along the top of the oil-pipe. The oil for some distance up from the bottom of the pipe may be at a much lower temperature, particularly along the walls of the pipe. in handling petroleum oils, the overheated oil is not injured and the only waste is of the additional power required for pumping the lowered capacity of the line with the oil of higher viscosity or the additional heat necessary for this overheat in order to warm all of the oil to the desired low viscosity. In other cases, particularly with food products, even as light overheat might be objectionable.

in those cases where the Reynolds number of the oil-pipe system indicates that the oil is in turbulent flow, the normal turbulent and convective motions within the fluid will tend to distribute the overheated oil and the underheated oil and to give a uniform temperature across the pipeline. There is always viscous flow adjacent the oil-pipe wall; and the extension inwardly of the layer of viscous oil reduces appreciably the effective capacity of the pipeline unless considerable more heat is applied than should be required. Particularly is this important at the start-up of an oil-pipe filled from previous operation with viscous oil, now thoroughly chilled; and there may always be some relatively stagnant oil near the bottom of an oil-pipe heated only at the top.

A multiplicity of heat-tubes in substantially axial parallel relation has been used to attempt to minimize these disadvantages and the distance which heat will have to be transferred either through the pipe wall or through the fluid, so that a more or less uniform minimum temperature is reached. Even so, with as many as six heat-tubes distributed around a 32-inch pipeline, as in the prior art, the oil flowing next the inner wall may be at a distance over 8 inches away from a heat-tube; and at least some part of the oil at such distances from a source of heat will never be adequately heated and will be in viscous flow. If there is adequate heat input to heat all of the oil, some oil is therefore heated to a higher temperature than necessary, which thus wastes electric heat.

Direct use of low-voltage heating, while sometimes possible, has many attendant disadvantages in heating small size oilpipes for short distances, although it does secure uniform heating of the oil in contact with all of the pipe, since it passes the length of the pipeline uniformly distributed. The current flow here, as in any conventional conductor-resistor, is the shortest path, end to end, along an element.

It has now been found possible to overcome this problem of minimizing fluid friction in the oil-pipe, while heating uniformly and economically all oil flowing immediately adjacent the pipe wall, and in pipes of any commercial size, 48 inches or more, and many miles in length. This is done by winding the heat-tube spirally around the oil-pipe. Unexpectedly, it has been found possible to force the AC to flow the much longer spiral path, even though the heat-tube, in part at least, may be a part ofthe pipe wall.

While in the past, spiral winding of pipes by electric resistance heating elements, well insulated from the pipe, was not unknown, it is quite against the usual laws controlling electrical conductivity to find that all of the AC can actually be made to travel a path in the metallic conductor, a steel pipe, which is very much longer, as is the spiral, than the shortest path which is along an element of the length of the pipe. This spiral heating system is valuable with any heat flux used, but particularly valuable with the high heat fluxes of the present invention.

if the helix of the heat-tube has a pitch (distance along the pipe between two turns) equal to the diameter D, its length is the hypotenuse of the right triangle, one side being D, along the element, and the other rri), the circumference. The heattube length per turn i s/ DQ17 11 OIYW, which is about 3.3D.

Thus, it has now been found possible, using the skin-effect, in a heat-tube which in the preferred forms of this invention actually includes some part of the surface of the oil-pipe, to make the AC flow about 3.3 times as far as the shortest possible distance (an element) as would be expected, and as is the case when the pipe wall itself is used as the conductor-resistor. Even greater ratios than 3.3 have been found to be possible with pitches less than the pipe diameter.

From this spiral or helical heat-tube, any given angle of the peripheral or circular arc of the pipeline-and especially the oil which flows past itwill receive throughout the length of the oil-pipe the same amount of heat as every other equal angle of circular arc. This contrasts with the nonuniform heating of those heat-tubes which are in a substantially axially parallel relation to the oil-pipe along a single element of the cylinder of the oil-pipe. Much less temperature difierences are present in the oil than when the oil is heated along one or more elements, with less overheated oil and lower heat costs. In very large oil-pipes in severe conditions, this spiral winding may be applied in a double helix; and a triple helix allows the use of three phases of a standard alternator.

Since most of the friction to fluid flow which must be overcome by the pumps is in the slip of the oil layers immediately adjacent the pipe wall of whatever size it follows that a unifonn heating of the oil next to the pipe wall reduces friction considerably, even if the oil in the center of the pipeline is still relatively cold. The converse of this shows why a heat-tube along the axis, besides giving additional surface or wall friction, has been found more expensive of heat, since the coldest oil-that near the surface-must be heated by heating to a higher temperature than oil which is in the center, and then passing this heat to the outer wall, cooled by the ambient conditions.

The oil-pipe of FIG. has a spirally wound heat-tube which may be applied in any one of several ways to its external surface as shown in FIGS. 1 to 4. Oil flowing near the surface crosses all of the paths of the turn pipe the helix of the heattube and thus is uniformly heated all around; its viscosity is reduced; and it flows with a minimum of friction. Since it will be uniformly heated around the tube, all of friction, interior oil, found more at start-up, will flow as a cylinder or plug" surrounded by this uniformly less viscous layer, which greases" the flow of the inner plug of cold oil. It has been found that even before ambient bulk of the oil in the center has been heated up to the temperature of that of the balance of the oil, the amount of oil pumped for a given pressure drop of the pipeline is not substantially less than after the oil is thoroughly heated throughout.

The flow capacity of the oil-pipe with a spiral heat-tube, as opposed to those with axially parallel heat-tubes on the surface, is always appreciably greater because of this same uniform reduction of the viscosity of the oil near the inner wall. This is with the same pressure exerted by the pump and the same heat input from the AC. The excellent transfer of heat by the crossflow of oil over the area of the oil-pipe immediately adjacent the heat-tube allows an unusually large heatflux to be generated and usedeffectively by this system.

With the improved designs of heat-tubes of this invention and conforming to, or integrally built into the walls of the oil-pipe, a much higher heat flux may be safely attained.

It is possible to apply the heat-tubes, constructed as in FIGS. 1(a) and FIG. 2(b) in a spiral winding.

if the two-duct pipe of FIG. 3 is twisted, after fabrication, while heated, a controlled number of turns for a fixed length, this spiral construction may also be used.

In the production of spiral pipe by welding a ribbon of skelp around a mandrel, it is also possible to wind in between the two adjoining edges being welded, a heat-tube as is done in building the heat-tube into the wall of a pipe along the element, as in FIG. 4(a) and (b). This may often be the cheapest production method for making oil-pipe with a built-in heattube, because of some advantages in economy of production of spiral wound pipe.

Spiral heat-tubes may be placed on a double or triple thread helix, by any of these methods, as may be necessary or advantageous to secure the desired heat effect simultaneously; and three such heat-tubes allow the balancing of the electrical circuitry with a three-phase alternator.

It has seldom been found necessary to space heat-tubes in this helix with a distance apart or pitch less than the diameter of the pipe.

In FIG. 5, the pitch, or distance apart of the turns of the helix is equal to twice the diameter of the pipe. As only one example of the preferred systems of fabrication of spiral-wound heat-tubes, and as shown in FIGS. 1 to 4, the spiral heat-tube, 2, is formed in the yvelding between the edges of the skelp, as indicated in FIG. 3(3) for a heat-tube, 2, along an element of the oil-pipe. The outer surface of the heat-tube is approxilight dashed lines. After welding, any part of the heat-tube which protrudes above the surface may be flattened to the outer wall of the oil-pipe. This construction is most eflicient for heating because of the turbulence set up in the flow of fluid over the heat-tube, and allows the maximum heat flux. However, it has and increased friction factor for the pipe, and will increase the required pressure on the pumps for a given throughout of oil. Alternately, the heat-tube may be welded into the pipe wall with its center at the midpoint, comparable to FIG. 4(a), tube 2, to reduce friction of the fluid flow; or it may be welded with its inner surface flush with the inner surface of the pipe, so that the disturbance to fluid flow is removed. This last construction is not detailed in the figures, but is obvious therefrom; and it is a preferred system.

With the helical pitch equal to the diameter of the oil-pipe, the helical heat-tube will have about 3.3 feet of heat-tube per running foot of oil-pipe. With a double spiral, it will be twice this. Because of the greater efficiency of heat transfer, a single helix spaced on an even greater pitch than the length of the diameter may usually be used on any size oil-pipe up to at least 48 inches diameter; and on all sizes of oil-pipes, a lower power consumption for pumping may be achieved for the reasons indicated, than with heat-tubes in axial parallel relation to the oil-pipe. This is particularly true when the heat-tube is constructed with its inner surface flush with the inner surface of the oil-pipe.

Thus, for a 48-inch oil-pipe, tests show that, to maintain an oil temperature of F. inside the pipe, with an outside temperature of 50 F, and a wind of 50 miles per hour, with linch thick glass fiber insulation having a density of 7 pounds per cubic foot, there is required approximately 175 watts of electric heating per lineal foot of the oil-pipe. A heat-tube in a spiral will have a greater length to supply this same heat flux, thus if the angle of a single heat-tube is a 30 with the elements of the pipe, each foot of heat-tube must supply about watts. The heat-tube may be made of schedule 40 steel pipe, and a size would be selected from the table:

DIMENSIONS OF SOHEDU1I;IEII 130 U.S. STANDARD STEEL The choice of the size of the heat-tube depends on the length, the size electric wire to be used, and its insulation If the length of the pipeline to be heated by a single spirally wound heat-tube is ID miles, 52,800 feet, the length of the heat tube at 30 to an element is 61,000 feet.

The heat-tube may be as large as 3%-inch US. Standard Pipe with dimensions as above. A single or multiple stranded copper cable equivalent to 0000 copper wire may be used. This 0000 copper wire has a diameter of 0.46 inches, a cross section of 21 1,600 circular mills, or 0.0662 square inches, and a resistance of 0.06 ohms per 1,000 feet at its operating temperature of about F. The weight is 0.640 pounds copper per foot.

if less expensive aluminum wire is used, it will require a cross section of about 0.221 square inches, with an equivalent diameter of about 0.53 inches and a weight of about 0.26 pounds per foot.

In addition to the usual advantages of aluminum of economy and greater flexibility, it has here the advantage of a larger diameter and hence more surface to transmit the heat of its line loss to the enclosing tube.

Using either copper or aluminum conductors of the respective sizes, there is required 618 amperes at 14,800 volts.

A built-up insulation of 0.075-inch thickness gives a copper cable of about %-inch diameter or an aluminum cable of about 1 1/ l 6-inch diameter. If the tube is filled with oil, the pulling of the cable will be easier, the electrical insulation will be better, and the thermal conductivity of heat from line losses will be better, i.e., a lower temperature of the conductor.

The choice of heat-tube size, cable, wire size, and insulation type and thickness depends on several considerations of design and construction of the complete assembly. A somewhat thicker insulation with more protective outer layers might be used. Also, a smaller, down to l /z-inch pipe size, possibly of heavier wall than 40 schedule pipe size would be indicated, depending on factors involved in the particular installation.

MONITORING LEAKS IN HEAT-TUBES The heat-tubes of the prior art have, in some cases, been filled with an inert gas under a pressure which is indicated and I recorded to note any dimunitions of the pressure due to leaks. The presently improved heat-tubes may likewise be so inspected and monitored for leaks by such a system, using inert L30 system of heat-tubes attached to oil-pipes has been found in those installations in which the heat-tube or heat-tubes have all joints made very tight, including those for inlet of electric wires. In FIG. 6 the heat-tube is diagrammed as s simple tube 2 of any cross section and is not shown as connected to an oilpipe, as it normally would be. The insulated conductor 3 passes through a vacuum tight joint of any standard type and is connected to an alternator 7, which supplies the AC. Another electrical connection 13 from the alternator passes through another vacuum tight connection 20 to the inside of the heat-tube to complete the circuit through the inside wall of the heat-tube. A tubular connection 9 from the heat-tube connects to a vacuum gauge 10 and a vacuum pump 11, which may be shut off from the heat-tube system by valve 12.

As shown schematically in FIG. 6, a vacuum is then produced inside the heat-tube by pump 11, and continuously monitored by watching pressure gauge 10. An increase of pressure indicates a leak. It may be that an absolutely vacuumtight system cannot be secured, and some leakage maybe noted continuously, which must be taken care of intermittently or continuously by the vacuum pump. Any abrupt change in such more or less constant leakage is, however, immediately noticeable on the monitoring vacuum gauge. Steps are then taken to find and repair the leak. The repair of the system against the vacuum leak-from outside air, in-may be more readily done by heavy paints or other resins applied in a more or less fluid condition to the surface of the joints, while the repair of the system against even minor pressure leaksfrom inside, out-may be very much more difficult, often involving shutdown of the heat-tube and the oil-pipe for extensive wire pulling, welding, etc.

Yet another system of monitoring the tightness of the heattube connections depends on keeping the tube around the insulated electric wire always under a substantial pressure with a low-viscosity oil. The system supplying oil under pressure is then indicated by 11. The oil gives additional insulation, aids in drawing the wire, and aids in transfer of heat from the internal conductor due to line losses. If the maintained oil pressure falls off of pressure gauge 10, a leak is indicated. If the leak is inwardly to the pipe and a petroleum oil, particularly a crude, is being transported, no damage is done, if the low-viscosity oil is a petroleum distillate.

HEAT-TUBES WITH VARYING INTERNAL PERIMETERS example, in FIG. 7, wherein the heat-tube alone is indicated without reference to the oil-pipe to which it may be integrally I attached, methods shown in (a) and (b) the sections of the heat-tube, all of the same outside diameter, but of several different internal diameters of the different sections l7, l8, and 19 give different resistances to the flow of AC. Each of these distances may be assumed to be very long in relation to the outside diameter. Thus, section, 19, of largest internal diameter would have the least efiective resistance per unit length to .the skin-efiect flow of AC, and would therefore generate less heat per unit length in that part of the heat-tube where less heat was needed. For the same reason, the section, 18, would generate more heat per unit length and the section I7, of smallest internal diameter would develop most.

As in other cases, the insulated electric wire, 3, connected to one terminal of the AC, goes through the entire length of the heat-tube. It connects at point 16, near the far end. Another wire 13 connects the near end of the heat-tube to the other AC terminal at point 15. Both points 15 and 16 are desirably on the inner surface of the tube wall, but the connections may be either through the wall or on the outside surface. Such a connection gives a very local point of live AC which must be insulated, but does not interfere with grounding the heat-tube a few feet away.

If attached to an oil-pipe, a heat-tube such as this, with different internal diameters, would be specified to be in those zones where ambient temperature was higher or lower, where more heat was lost due to the oil-pipe being buried in the ground, submerged in water, etc. If the same intensity of AC passed through each, the section where greater resistance was required would be placed where most heat was required.

The outer diameter of the heat-tube is not important if the wall thickness is more than about twice 6 the depth of penetration of the induction and magnetic effects. For most mild steels, 8 has been found to be about 0.04 inch.

In this use of the heat-tube, in zones ofdifferent heat losses, the outside diameter is maintained constant in FIG. 7. Standard steel pipe of the same outside diameter, but different wall thicknesses, is used. The skin-effect is determined by the inside perimeter, thus there will be a greater electrical resistance for the tube with the thicker wall and the most metal in the 6 5 cross section.

DIMENSIONS OF U.S. STANDARD PIPE Cross-section areas Outside Wall Inside Inside diameter thickness diameter Flow Metal perimeter scheigisile No.: h

nc .533 sq. in .333 sq. in 2.588 inches. .742 inch .432 sq. in .433 sq. in 2.331 inches. .612 inch .294 sq. in .572 sq. in-.. 1.922 inches. X .434 inch .148 sq. in. 718 sq. in 1.363 inches. Baum" 1.90 3. .463 1.9. controls" Flow of fluids-.- Area for usual Area for skin strength. electrical effect conducconduction. tivity of A.C.

This may well be illustrated by comparison of the dimensions of different weights of %-inch U.S. Standard Steel Pipe. The following table has been prepared for those weights of sufficient wall thickness to be used as heat-tubes, also the ratios of the important dimensions of the heaviest to the lightest. Also indicated are the dimensions which control bursting strength, flow of fluids in the usual use of the pipe,

usual electrical resistance, and electrical resistance to AC flow controlled by the skin-effect from an internal conductor.

The tube with the largest inside diameter a'nd'm least wall thickness has, compared with the one with the smaller inside diameter and the greatest wall thickness: (a) much more area for flow of liquids (3.6 times), (b) much less area for usual electrical conductance (0.463 times), and hence greater electrical resistance (reciprocal of 0.463=2.l6 times), while having (c) much greater perimeter for skin-effect flow of AC 149 times) when there is an internal conductor, and hence a correspondingly lower resistance to flow of AC (reciprocal of l.9=0.526 times). Some of the novelties of the present invention are well demonstrated by consideration of these figures.

While this use of several heat-tubes in series of different ef fective resistances, and hence different heat productions per unit length, is particularly advantageous for use with the large heat fluxes of the present invention, it is also possible to use this system with the lower heat fluxes of the prior art.

PAIRS OF HEAT-TUBES IN LONG LENGTHS FIG. 8 diagrams the installation of an oil-pipe of major length for which it is desired to keep the stations for supply of electrical energy to a minimum number. It has been found that the suitable or economical maximum length of a heat-tube system may be from 15 to 50 miles, depending on the various conditions peculiar to the particular installation. However, in long lines, periodically there must be a repeater" station, usually at distances apart equal to the length of a heat-tube.

However, with electrical connections shown in FIG. 8, the repeater stationsmay be at twice the distance apart as the economical maximum length of the heat-tube. In the long oilpipe, I, a pair of heat-tubes, 2" and 2", are placed in opposite lengths from one supply of AC. The two electric wires inside the heat-tubes, 2 and 2" run in opposite directions, and are each attached to a terminal of the AC, while the terminals connect to the return legs of the circuit from the near end of the heat-tube, usually but 'not necessarily from a point on its, inner surface.

Similarly, at another repeater station, not shown, on the left, connections are made from another pair of heat-tubes, one of which is 2. The ends of heat-tubes 2' and 2", back up to each other. Normally, there is no need for electrical insulation of one section of the pipeline from the other at this point; and all lengths of the heat-tubes are always grounded. Heat-tubes 2" and 2"" back up to each other; and 2" would be one of a pair serviced by a repeater station not shown on the right.

The optimum length for a heat-tube to be used is fixed by design conditions. Then, by placing alternate heat-tubes in opposing relation to each other, as in FIG. 8, there will only need to be one-half as many junctions for supply of electric current, as if the usual single directional system was applied. In long pipelines, repeater stations are usually installed at the booster pumping stations for the oil, where there is also an alternator feeding AC in both directions.

The greatest heat requirement is at the startup after a shutdown, with the pipe filled with cold oil. If storage tanks are available at each station, the oil-pipe on the downstream side is supplied with the full AC capacity of the alternator. As one heat-tube may not take the increased input of AC a second heat-tube is incorporated in the pipeline to give double normal. heat for this start up. (It is also a spare unit in case of damage to the first.) After the downstream line is heated, it is put in service to pipe oil into the storage tank, the upstream pipe is then heated with as much of the total capacity as can be afforded, while the section beyond is being heated from the next alternator station. Thus, the entire line is started progressively.

Iclaim:

I. In the system for heating a fluid being transported in a pipe comprising:

a. a fluid transport-pipe;

b. at least one elongated heat-tube coextensive in length with at least a section of said pipe, said tube being of a metal having magnetic properties and conducting electricity, said heat-tube being secured in heat exchange relation with and having a substantial part of its wall in common with the wall of said transport-pipe;

c. a source of AC having a first terminal and a second terminal;

d. an electrical connection between the first terminal of the source of the AC and the one end-of said heat-tube;

e. an electrical conductor extending for a substantial distance through the inside of said heat-tube from said one end to the other and insulated electrically therefrom;

f. an electrical connection betweenthe second terminal of the source of the AC and the end of' the said electrical conductor which is near the electrical connection of said heat-tube and said first terminal of the source of AC g. an electrical connection between the said heat-tube and the said electrical conductor remote from its electrical connection with the said source of AC;

h. an AC circuit thus established from the said second terminal of the said AC source through the substantial distance of the said electrical conductor inside the heattube, then back through the said heat-tube, so as to produce a skin effect current on the inner surface of the said heat-tube, thus producing heat; and said AC circuit being completed back to the first terminal of the source of the AC; whereby i. a substantial part of the said heat produced is transferred tosaid fluid through said wall which is common to both said heat-tube and said transport-pipe.

2. In the system of claim 1 wherein the said fluid transportpipe and the said heat-tube are constructed of steel.

3. In the system of claim I wherein:

a. the transport-pipe is of cylindrical and the internal surface of said heat-tube comprises a portion of the external cylindrical surface of said transport-pipe between two longitudinal straight line elements generating said cylindrical surface and the inner surface of a strip of steel which covers said external surface of the transport pipe between said straight line generating elements thereof;

. said strip of steel is formed as a concave trough of radius of curvature substantially less than that of the normal outside surface of the said transport pipe; and

. the longitudinal edges of said concave trough are firmly contacted against said two straight line generating elements of the said transport-pipe to make an electrical connection between said concave trough and said transport-pipe and tto encompass the said external surface between.

4. In the system of claim 3, wherein the longitudinal edges of the said concave trough are welded to the said transportpipe along the said longitudinal straight line elements generating said cylindricalsurface of the said transport-pipe which said edges are in contact with the said transport-pipe.

5. In the system of claim 3 wherein at least a part of the outer surface of the said transport-pipe between the said two longitudinal straight line elements generating said .cylindrical surface and defining the internal surface of the heat-tube is flattened.

6. In the system of claim -1, wherein:

a. the transport-pipe is cylindrical and the internal surface of the said heat-tube comprises a part of the external cylindrical surface of a transport-pipe between two straight line elements generating the cylindrical surface and the innersurface of a strip of steel which covers said part of the external surface of the transport-pipe;

. at least a part .of the external surface of said transportpipe comprising said internal surface of the heat-tube is formed to provide a generally longitudinal inwardly directed groove of size to accommodate at least in part the said electric conductor extending inside said heattube; and I c. the longitudinal edges of said strip of steel are firmly contacted against said two straight line elements of the said transport-pipe, to encompass the said external surface between.

7. In the system of claim 1 wherein the said heat-tube is integrally formed with said transport-pipe as one of two channels formed in the manufacture of a transport-pipe, the second of said two channels being used for transport of said fluid being heated.

8. In the system of claim 1, wherein the said heat-tube is welded between the two edges of at least one piece of formed skelp to become an integral part of the wall of said transportpipe, which results from the joining of the edges of the skelp to the heat-tube.

9. In the system of claim 8, wherein at least two such heattubes are welded between edges of skelp in the same length of said transport-pipe, there being a piece of skelp between adjacent heat-tubes.

10. In the system of claim 8, wherein said heat-tube is welded between the two edges of said skelp so that it extends into said transport-pipe with at least a part of the exterior surface of said heat-tube being in contact with the fluid flowing in said transport-pipe.

11. In the system of claim 10, wherein said heat-tube is made in the form of a helix winding around and included as a part of said wall of said transport-pipe.

12. In the system of claim 8, wherein a part of the said heattube extends outside of the surface of the said transport-pipe after the said welding and is pressed inwardly to make its surface conform more nearly to that of the transport-pipe.

13. In the system of claim 8, wherein the said heat-tube is made in the form of a helix winding around and included as a part of said wall of said transport-pipe.

14. In the system of claim 1, wherein the said heat-tube is in the form of a helix winding around said transport-pipe.

15. In the system ofclaim 1, wherein:

a. the internal surface of said heat-tube comprises a portion of the external cylindrical surface of said transport-pipe between two uniformly spaced helices generated on the external surface thereof and the inner surface of a strip of metal having magnetic properties and conducting electricity which overlies said electrical conductor and covers said external surface of said transport-pipe between the two uniformly spaced helices thereon;

b. said strip is formed as a concave trough of radius of curvature substantially less than that of the outside surface of the said transport-pipe; and

c. the longitudinal edges of said concave trough are firmly contacted against said two uniformly spaced helices to make an electrical connection with said transport-pipe and to encompass its external surface between said helices.

16. In the system of claim I5. wherein said transport-pipe and said strip are of steel and are firmly contacted by welding the edges of said strip to the said external surface of said transport-pipe along said helices.

17. In the system of claim 1, wherein the said heat-tube, including the point of entrance of said electrical connection to said electrical conductor extending for a substantial distance inside said heat-tube, is made vacuumtight, and said heat-tube is connected to a vacuum system comprising a pressure gauge, a vacuum pump, and a valve allowing the vacuum pump to be shut off from the heat-tube, said pressure gauge indicating subatmospheric pressure within the said heat-tube after evacuation by said vacuum pump; and increase in said atmospheric pressure after said evacuation and after closing said valve indicating a leak in said heat-tube.

18. In the system of claim 1, wherein the space inside the heat-tube and surrounding the said electrical conductor ex tending for a substantial distance therein is filled with a material nonconductive to electricity comprising an oxide of an alkaline earth metal.

19. In the system of claim 1 wherein:

a. the said transport-pipe provided with at least one pair of heat-tubes distributed along its length, heating respective sections thereof and longitudinally numbered alternately odd and even;

b. the said odd-numbered and even-numbered heat-tubes of a pair running in opposite directions from ends which are adjacent; said transport-pipe over a length equal to the sum of the lengths of each pair of heat-tubes, one oddnumbered, and one even-numbered.

20. In the system of claim 1, wherein the said heat-tube, including the point of entrance of said electrical connection to said electrical conductor extending for a substantial distance inside said heat-tube, is made tight against fluid pressure; and said heat-tube is connected to a supply of oil under pressure which is indicated by a pressure gauge so that when said pressure gauge shows a decrease in said pressure of said oil within the said heat-tube, a leak in said heat-tube is indicated.

21. In the system of claim 1, wherein each heat-tube comprises a plurality of sections arranged end-to-end and each section having a different internal perimeter.

22. A system for heating materials being transported comprising:

a. an elongated pipe with walls of a heat-conductive materi' al in which said materials are being transported;

b. an elongated heat-tube of ferromagnetic material formed in part by a generally elongated portion of the surface of the wall of said elongated pipe and in part by an elongated shaped strip of ferromagnetic material which overlies said generally elongated portion of said elongated pipe;

0. means for attaching the edges of said elongated shaped strip forming part of the wall of the heat-tube throughout its length tightly against the generally elongated portion of said surface of said elongated pipe so as to form a contact angle at least with said surface;

d. an electrical connection between a source of AC and a first end of said heat-tube;

e. an electrical conductor extending through the inside of said heat-tube from said first end to the other end thereof and insulated electrically from said heat-tube;

f. an electrical connection between the source of the AC and said end of the said electrical conductor which is near the said first end of said heat-tube;

g. an electrical connection between the said electrical conductor and the other end of said heat-tube;

h. whereby an AC circuit is thus established from the said AC source through the said electrical conductor inside the heat-tube, then back through said heat-tube to the source of the AC; thus i. producing a skin effect current on the inner surface of the said heat-tube so as to develop heat which passes to said elongated pipe and said materials being transported.

23. A heating system as in claim 22, wherein the part of the outer wall of said elongated pipe forming part of the heat-tube is flattened in at least a part of said generally elongated portion.

24. A heating system as in claim 22, wherein said elongated pipe is of generally cylindrical shape and the part thereof used in forming said heat-tube is the convex outer surface thereof.

25. A heating system as in claim 22, wherein said elongated shaped strip completing said heat-tube overlies the outside of said elongated pipe and has a generally concave inner surface.

26. A heating system as in claim 22, wherein said part of said elongated pipe forming said heat-tube is indented inwardly throughout the length of the inner wall of said elongated pipe to form an open channel, said elongated shaped strip which completes the heat-tube being fastened to the outer wall of said elongated pipe so as to cover the open channel formed by the indent.

27. A heating system as in claim 22, wherein said elongated shaped strip completing the heat-tube overlies and is connected integrally and throughout its length across a portion of the inner wall of said elongated pipe so as to form two channels, a major one for the transport-pipe and a minor one for the heat-tube.

28. A heating system as in claim 22, wherein said elongated shaped strip completing the heat-tube is connected integrally and throughout its length across a portion of the inner wall of said elongated pipe.

29. A heating system as in claim 28, wherein said elongated shaped strip has a generally convex surface on the side in contact with said materials being transported.

30. A system as in claim 22, wherein said heat-tube follows 

1. In the system for heating a fluid being transported in a pipe comprising: a. a fluid transport-pipe; b. at least one elongated heat-tube coextensive in length with at least a section of said pipe, said tube being of a metal having magnetic properties and conducting electricity, said heat-tube being secured in heat exchange relation with and having a substantial part of its wall in common with the wall of said transport-pipe; c. a source of AC having a first terminal and a second terminal; d. an electrical connection between the first terminal of the source of the AC and the one end of said heat-tube; e. an electrical conductor extending for a substantial distance through the inside of said heat-tube from said one end to the other and insulated electrically therefrom; f. an electrical connection between the second terminal of the source of the AC and the end of the said electrical conductor which is near the electrical connection of said heat-tube and said first terminal of the source of AC g. an electrical connection between the said heat-tube and the said electrical conductor remote from its electrical connection with the said source of AC; h. an AC circuit thus established from the said second terminal of the said AC source through the substantial distance of the said electrical conductor inside the heat-tube, then back through the said heat-tube, so as to produce a skin effect current on the inner surface of the said heat-tube, thus producing heat; and said AC circuit being completed back to the first terminal of the source of the AC; whereby i. a substantial part of the said heat produced is transferred to said fluid through said wall which is common to both said heat-tube and said transport-pipe.
 2. In the system of claim 1 wherein the said fluid transport-pipe and the said heat-tube are constructed of steel.
 3. In the system of claim 1 wherein: a. the transport-pipe is of cylindrical and the internal surface of said heat-tube comprises a portion of the external cylindrical surface of said transport-pipe between two longitudinal straight line elements generating said cylindrical surface and the inner surface of a strip of steel which covers said external surface of the transport-pipe between said straight line generating elements thereof; b. said strip of steel is formed as a concave trough of radius of curvature substantially less than that of the normal outside surface of the said transport pipe; and c. the longitudinal edges of said concave trough are firmly contacted against said two straight line generating elements of the said transport-pipe to make an electrical connection between said concave trough and said transport-pipe and to encompass the said external surface between.
 4. In the system of claim 3, wherein the longitudinal edges of the said concave trough are welded to the said transport-pipe along the said longitudinal straight line elements generating said cylindrical surface of the said transport-pipe which said edges are in contact with the said transport-pipe.
 5. In the system of claim 3 wherein at least a part of the outer surface of the said transPort-pipe between the said two longitudinal straight line elements generating said cylindrical surface and defining the internal surface of the heat-tube is flattened.
 6. In the system of claim 1, wherein: a. the transport-pipe is cylindrical and the internal surface of the said heat-tube comprises a part of the external cylindrical surface of a transport-pipe between two straight line elements generating the cylindrical surface and the inner surface of a strip of steel which covers said part of the external surface of the transport-pipe; b. at least a part of the external surface of said transport-pipe comprising said internal surface of the heat-tube is formed to provide a generally longitudinal inwardly directed groove of size to accommodate at least in part the said electric conductor extending inside said heat-tube; and c. the longitudinal edges of said strip of steel are firmly contacted against said two straight line elements of the said transport-pipe, to encompass the said external surface between.
 7. In the system of claim 1 wherein the said heat-tube is integrally formed with said transport-pipe as one of two channels formed in the manufacture of a transport-pipe, the second of said two channels being used for transport of said fluid being heated.
 8. In the system of claim 1, wherein the said heat-tube is welded between the two edges of at least one piece of formed skelp to become an integral part of the wall of said transport-pipe, which results from the joining of the edges of the skelp to the heat-tube.
 9. In the system of claim 8, wherein at least two such heat-tubes are welded between edges of skelp in the same length of said transport-pipe, there being a piece of skelp between adjacent heat-tubes.
 10. In the system of claim 8, wherein said heat-tube is welded between the two edges of said skelp so that it extends into said transport-pipe with at least a part of the exterior surface of said heat-tube being in contact with the fluid flowing in said transport-pipe.
 11. In the system of claim 10, wherein said heat-tube is made in the form of a helix winding around and included as a part of said wall of said transport-pipe.
 12. In the system of claim 8, wherein a part of the said heat-tube extends outside of the surface of the said transport-pipe after the said welding and is pressed inwardly to make its surface conform more nearly to that of the transport-pipe.
 13. In the system of claim 8, wherein the said heat-tube is made in the form of a helix winding around and included as a part of said wall of said transport-pipe.
 14. In the system of claim 1, wherein the said heat-tube is in the form of a helix winding around said transport-pipe.
 15. In the system of claim 1, wherein: a. the internal surface of said heat-tube comprises a portion of the external cylindrical surface of said transport-pipe between two uniformly spaced helices generated on the external surface thereof and the inner surface of a strip of metal having magnetic properties and conducting electricity which overlies said electrical conductor and covers said external surface of said transport-pipe between the two uniformly spaced helices thereon; b. said strip is formed as a concave trough of radius of curvature substantially less than that of the outside surface of the said transport-pipe; and c. the longitudinal edges of said concave trough are firmly contacted against said two uniformly spaced helices to make an electrical connection with said transport-pipe and to encompass its external surface between said helices.
 16. In the system of claim 15, wherein said transport-pipe and said strip are of steel and are firmly contacted by welding the edges of said strip to the said external surface of said transport-pipe along said helices.
 17. In the system of claim 1, wherein the said heat-tube, including the point of entrance of said electrical connection to said electrical conductor extending for a substantial distance inside said heat-tube, is made vacuumtight, and said heat-tube is connected to a vacuum system comprising a pressure gauge, a vacuum pump, and a valve allowing the vacuum pump to be shut off from the heat-tube, said pressure gauge indicating subatmospheric pressure within the said heat-tube after evacuation by said vacuum pump; and increase in said atmospheric pressure after said evacuation and after closing said valve indicating a leak in said heat-tube.
 18. In the system of claim 1, wherein the space inside the heat-tube and surrounding the said electrical conductor extending for a substantial distance therein is filled with a material nonconductive to electricity comprising an oxide of an alkaline earth metal.
 19. In the system of claim 1 wherein: a. the said transport-pipe provided with at least one pair of heat-tubes distributed along its length, heating respective sections thereof and longitudinally numbered alternately odd and even; b. the said odd-numbered and even-numbered heat-tubes of a pair running in opposite directions from ends which are adjacent; said transport-pipe over a length equal to the sum of the lengths of each pair of heat-tubes, one odd-numbered, and one even-numbered.
 20. In the system of claim 1, wherein the said heat-tube, including the point of entrance of said electrical connection to said electrical conductor extending for a substantial distance inside said heat-tube, is made tight against fluid pressure; and said heat-tube is connected to a supply of oil under pressure which is indicated by a pressure gauge so that when said pressure gauge shows a decrease in said pressure of said oil within the said heat-tube, a leak in said heat-tube is indicated.
 21. In the system of claim 1, wherein each heat-tube comprises a plurality of sections arranged end-to-end and each section having a different internal perimeter.
 22. A system for heating materials being transported comprising: a. an elongated pipe with walls of a heat-conductive material in which said materials are being transported; b. an elongated heat-tube of ferromagnetic material formed in part by a generally elongated portion of the surface of the wall of said elongated pipe and in part by an elongated shaped strip of ferromagnetic material which overlies said generally elongated portion of said elongated pipe; c. means for attaching the edges of said elongated shaped strip forming part of the wall of the heat-tube throughout its length tightly against the generally elongated portion of said surface of said elongated pipe so as to form a contact angle at least 90* with said surface; d. an electrical connection between a source of AC and a first end of said heat-tube; e. an electrical conductor extending through the inside of said heat-tube from said first end to the other end thereof and insulated electrically from said heat-tube; f. an electrical connection between the source of the AC and said end of the said electrical conductor which is near the said first end of said heat-tube; g. an electrical connection between the said electrical conductor and the other end of said heat-tube; h. whereby an AC circuit is thus established from the said AC source through the said electrical conductor inside the heat-tube, then back through said heat-tube to the source of the AC; thus i. producing a skin effect current on the inner surface of the said heat-tube so as to develop heat which passes to said elongated pipe and said materials being transported.
 23. A heating system as in claim 22, wherein the part of the outer wall of said elongated pipe forming part of the heat-tube is flattened in at least a part of said generally elongated portion.
 24. A heating system as in claim 22, wherein said elongated pipe is of generally cylindrical shape and the part thereof used in forming said heat-tube is the convex outer surface thereof.
 25. A heating system as in claim 22, wherein said elongated shaped strip completing said heAt-tube overlies the outside of said elongated pipe and has a generally concave inner surface.
 26. A heating system as in claim 22, wherein said part of said elongated pipe forming said heat-tube is indented inwardly throughout the length of the inner wall of said elongated pipe to form an open channel, said elongated shaped strip which completes the heat-tube being fastened to the outer wall of said elongated pipe so as to cover the open channel formed by the indent.
 27. A heating system as in claim 22, wherein said elongated shaped strip completing the heat-tube overlies and is connected integrally and throughout its length across a portion of the inner wall of said elongated pipe so as to form two channels, a major one for the transport-pipe and a minor one for the heat-tube.
 28. A heating system as in claim 22, wherein said elongated shaped strip completing the heat-tube is connected integrally and throughout its length across a portion of the inner wall of said elongated pipe.
 29. A heating system as in claim 28, wherein said elongated shaped strip has a generally convex surface on the side in contact with said materials being transported.
 30. A system as in claim 22, wherein said heat-tube follows a three dimensional curved path in being formed along the wall of said elongated pipe.
 31. A heating system as in claim 30, wherein said curved path is a helix.
 32. A heating system as in claim 22, wherein said heat-tube follows a helix in being formed along the wall of said elongated pipe. 